The invention relates to a digital microphone capable of directly converting a displacement of a diaphragm into a digital signal with the aid of optical means. A Hi-Fi sound recently has been desired in the field of audio. To this end, various kinds of record carrier have been proposed such as a direct cutting record; a PCM disc record capable of preventing a decrease in a sound quality due to intermediate manufacturing processes, for example, track-down and mixing, by effecting a pulse-code modulation for the electric signal derived from a microphone; and a digital audio disc on which a digital signal obtained by effecting a pulse-code modulation for the electric signal derived from a microphone is directly recorded. The tendency described above with respect to the disc record and digital audio disc is applicable to the field of a magnetic tape.
When an analogue sound signal is converted into a digital signal, there is the problem that a high-speed, high-accuracy and inexpensive analogue to digital converter (A-D converter) cannot be easily obtained. The presently available A-D converters satisfy the requirements for high-speed and high-accuracy, however the cost thereof is still high, which is an obstruction for the development of a digital recording in an audio system. One of the methods for resolving the above problem is to directly derive from a microphone a digital signal which designates the displacement of a diaphragm, which makes it possible to omit the A-D converter from the audio system.
A conventional microphone which has been proposed for that purpose is shown in FIG. 1. A diaphragm 1 made of a plastic film is arranged opposite to a plurality of fixed electrodes 2, 2', 2" . . . A metal layer 3 is applied on the surface of the diaphragm 1 by evaporation to form an electrode which is opposed to the fixed electrodes via an air-gap 4. A plurality of macromolecular films 5, 5', 5" . . . each of which constitutes an electret for applying an electrostatic field to the air-gap 4 are sticked on the surfaces of the fixed electrodes, respectively. The fixed electrodes 2, 2' . . . and films 5, 5' . . . are embedded in an insulator 6. In this way, a plurality of condenser microphone elements are formed. A plurality of comparators 7, 7', 7". . . are connected to the fixed electrodes 2, 2', 2" . . . of the microphone elements, respectively. A voltage is applied to the electrode 3 via a metal case 8. The respective output voltages from the condenser microphone elements are compared to the comparators with predetermined reference voltages which are different from each other in a stepwise manner. A series of output signals from the comparators 7, 7', 7" . . . constitute a binary code digital signal which designates the displacement of the diaphragm 1. In this manner the acoustic energy incident upon the diaphragm is converted into the digital signal.
FIG. 2 shows a part of the electric circuit included in the comparators of the known microphone of FIG. 1. It should be noted that FIG. 2 illustrates only three condenser microphone elements and the electric components related thereto in order to simplify the drawing. Each condenser microphone 9, 9' or 9" is connected to the related comparator 10, 10' or 10". Each voltage from the microphone element, 9, 9' or 9" is compared with the related voltage applied by an adder 12, 12' or 12". The adders are connected to the reference voltage sources, respectively, each value of which is denoted by V.sub.o, 2V.sub.o or 2.sup.2 V.sub.o. The switches 11, 11' and the adders 13, 13', 13" serve to add the reference voltage of the upper position to the reference voltage of the lower position. The switches 11 and 11' are operated according to the output signals from the comparators 10' and 10", respectively. If the output voltage of each microphone element 9, 9' or 9" is V which is in V.sub.o .ltoreq.V&lt;2V.sub.o, the binary code output signals from the output terminals A, B and C of the comparators 10, 10' and 10" are "1", "0" and "0", respectively. Therefore, the binary code output signal "001" may be obtained from the microphone. When the output voltage of each microphone is 3V.sub.o which is in 2V.sub.o .ltoreq.3V.sub.o &lt;2.sup.2 V.sub.o, the binary code output signals from the output terminals A, B and C are "1", "1" and "0", respectively. Consequently, the binary code output signal "011" may be obtained from the microphone.
As is apparent from the foregoing, these binary code output signals designate amounts of the displacement of the diaphragm due to an incident acoustic energy. However, such conventional microphone requires a complicated electric circuit so that the construction of the whole microphone is also complicated. Furthermore, the complicated process for manufacturing the diaphragm is necessitated.